The structure of organic electroluminescent devices (OLEDs) in which organic semiconductors are employed as functional materials is described, for example, in U.S. Pat. No. 4,539,507, 5,151,629, EP 0676461 and WO 98/27136. The emitting materials employed here are increasingly organometallic complexes which exhibit phosphorescence instead of fluorescence (M. A. Baldo et al., Appl. Phys. Lett. 1999, 75, 4-6). For quantum-mechanical reasons, an up to four-fold increase in energy and power efficiency is possible using organometallic compounds as phosphorescence emitters. In general, however, there is still a need for improvement in OLEDs which exhibit triplet emission, in particular with respect to efficiency, operating voltage and lifetime. This applies, in particular, to OLEDs which emit in the relatively short-wave range, i.e. green and in particular blue. Thus, no blue-emitting triplet emitters which meet the technical requirements for industrial use are known to date.
In accordance with the prior art, the triplet emitters employed in phosphorescent OLEDs are, in particular, iridium complexes. An improvement in these OLEDs has been achieved by employing metal complexes containing polypodal ligands or cryptates, as a consequence of which the complexes have higher thermal stability, which results in a longer lifetime of the OLEDs (WO 04/081017, WO 05/113563, WO 06/008069). However, these complexes are not suitable for blue emission, in particular for saturated deep-blue emission.
The prior art furthermore discloses iridium complexes which contain imidazophenanthridine derivatives or diimidazoquinazoline derivatives as ligands (WO 07/095,118). These complexes can result in blue phosphorescence on use in organic electroluminescent devices, depending on the precise structure of the ligand. Here too, further improvements are desirable with respect to efficiency, operating voltage and lifetime. In particular, there is also a need for improvement here with respect to the colour coordinates in order to be able to achieved deep-blue emission.